it shouldn't be that hard if you are doing it with live video feed (GoPro connected to your TV or PC monitor). when you start, don't screw the lens too far in, you don't want to hit the sensor, leave the lens sticking out little bit before working your way down while watching live feed on the monitor. it may be challenging to find a subject far away from you (110ft), but if you focus on any subject in your room (one that's furthest away from you) then get focused on that, then you should be almost 95% there. if it's difficult to find a subject further away (through a window looking outside) then you can just go outside and find something 110ft away and take a picture/video of that, turn the lens little bit (0.5mm CW and CCW) to take few more shots and maybe another 2 shots 1mm CW and CWW from the initial position, then check the video/photo from each lens position on your computer to find which one was the best focused.
when you do this, it's good practice to use white marker to make a dot for example at 12 o'clock on the housing of your lens (aligned with another white dot on the camera), that way you can see better when you are fine-tuning CW or CCW by 0.5mm or 1mm. once you found the sweet spot mark it again so that you always know where the lens is supposed to be for the best focused picture. (for example when installing or removing ND filter, it may happen that you accidentally turn the lens if it's not properly secured in the thread and you want to have an easy way to put the lens in your correct position if that ever happens).
